For many processes in the pulp and paper field it is desirable to be able to mix chemicals in fluid form (whether gaseous or liquid) into medium consistency pulp (typically pulp having a consistency of about 5-18%). In order to effectively do this, it is necessary that the pulp suspension (which is comminuted cellulosic fibrous material) be fluidized. This may be accomplished, for example, by causing the pulp with chemical to flow in an annulus while an impeller, which comprises one of the components defining the annulus and having lobes or vanes, is rotated at extremely high speed, a speed sufficient to effect fluidization. For example, see U.S. Pat. Nos. 4,339,206 and 4,577,974 and Canadian Patent 1,102,604. While such mixers do normally provide a suitable mixing action, because of the small residence time of the pulp and chemical in the fluid mixing zones, the efficiency of the mixing is not always as good as desired. Therefore, according to the present invention, the efficiency of a medium consistency mixer is desirably increased.
According to the present invention, a mixer, and method of mixing, suitable for use with medium consistency pulp are provided which increase the efficiency of the mixer compared to the conventional prior art by intensifying the turbulent action in at least one fluidized zone. This is accomplished, according to the present invention, by constantly changing the annular fluidization zone so as to subject the pulp to an unsteady-state shear field. According to the present invention, rather than subjecting the pulp to merely one field at a time, e.g., a field in the axial plane where the pulp velocity is a function of the cross section of the annulus, it is subjected to two transverse fields simultaneously. According to the invention, one shear field is generated in a radial plane where shear is a function of radius for a given rotational speed, while another shear field is contemporaneously generated in the axial plane. This unsteady-state shear field in two planes increases the mixing efficiency significantly.
According to one aspect of the present invention, a method of mixing a fluid with cellulosic pulp having a consistency of about 5-18% throughout mixing is provided which comprises the following steps: (a) Introducing the fluid and the pulp having a consistency of about 5-18% into a first fluidization annulus in a first fluidization zone. (b) In the first fluidization annulus in the first fluidization zone, fluidizing the pulp while subjecting the pulp and fluid to a constantly changing shear field simultaneously developed in both radial and axial planes; and (c) discharging the pulp, with mixed in fluid, from the first fluidization zone. A second fluidization zone may also be provided, either prior to the first zone, or after it, in which the pulp and fluid are subjected to a constantly changing shear field developed in substantially only one plane (a radial plane).
According to another aspect of the present invention, a mixer is provided, particularly (although not exclusively) for use in the method as described above. The mixer comprises: A housing having a first interior portion encompassing an axial plane, a second interior portion, a first inlet, a second inlet, and an outlet. A rotor. Means for mounting the rotor for rotation about a first axis, within at least the housing first interior portion, the first axis disposed in the axial plane. Means for rotating the rotor about the first axis. The housing first interior portion and the rotor configured so as to define a fluidization zone having a constantly changing configuration creating an ever changing shear field in the axial plane, and in radial planes substantially perpendicular to the axial plane; and the first inlet, second inlet, and outlet spaced so that two different fluids introduced into the fluidization zone by the first and second inlets are mixed before discharge of a mixed fluid through the outlet.
The configuration of the rotor of the mixer described above which results in the desired changing fields in an axial plane and radial plane substantially perpendicular to the axial plane comprises a varying cross section of the rotor along its length. Also, the housing first inner portion has a varying cross section substantially mimicking the varying cross section of the rotor. The rotor may have a disc at a first axial end thereof closest to the rotating means, with the second interior housing portion having a surface defining a fluidization zone with the disc.
The invention also comprises a rotor per se, utilizable in a mixer. The rotor according to the invention is unique in that it comprises a body element elongated in a dimension of elongation, and having an external surface with a continuously varying cross-sectional area along a major portion of the body element in the dimension of elongation. Preferably, this is provided by an external surface shaped to simulate a plurality of alternately oriented cone frustums. A plurality of vanes are connected to the body element, including portions of the vanes generally following the contour of the body element external surface. Finally, a means for connecting the rotor to a shaft is provided. Optionally, a disc may be disposed in a plane perpendicular to the dimension of elongation of the body element, the disc disposed adjacent the means for connecting the rotor to a shaft. Continuations of the vanes may be provided from the body element onto the disc (e.g., radially extending on the disc), an extension portion may extend axially from the body element in the dimension of elongation, from a second axial end opposite the connection to a shaft. The number of vanes and their position may vary widely, but in exemplary embodiment four evenly spaced parallel straight vanes may be provided.
It is a primary object of the present invention to provide for enhanced mixer efficiency, including utilizing a uniquely constructed rotor, and in a preferred embodiment for acting upon medium consistency pulp. This and other objects will become clear from an inspection of the detailed description of the invention and from the appended claims.